How to express a Hash + Range in JSON for dynamodb getItem on Node.js

I have a table called Message in dynamodb. The key is composed of a Hash (called entryStamp as a Number), and a Range ( userId as a String).

I'm trying to do a getItem on it using Node.js Lambda. I know how to getItem for a different table with a Hash only key, and it works.

But I can't find documentation on how to modify my json to express the Range part of the key.

var params = {
    "ConsistentRead": false,
    "Key": {
        HashKeyElement: { N: desiredSendStamp },
        RangeKeyElement: { S: userId }
           },
    "ProjectionExpression": "message",
    "ReturnConsumedCapacity": "NONE",
    "TableName": "Message"
}

Thanks in advance! When I send the above to getItem, I get this:

"errorMessage": "The provided key element does not match the schema",
"errorType": "ValidationException",
"stackTrace": [
    "Request.extractError (/var/runtime/node_modules/dynamodb-doc/node_modules/aws-sdk/lib/protocol/json.js:40:27)",
    "Request.callListeners (/var/runtime/node_modules/dynamodb-doc/node_modules/aws-sdk/lib/sequential_executor.js:100:18)",
    "Request.emit (/var/runtime/node_modules/dynamodb-doc/node_modules/aws-sdk/lib/sequential_executor.js:77:10)",
    "Request.emit (/var/runtime/node_modules/dynamodb-doc/node_modules/aws-sdk/lib/request.js:604:14)",
    "Request.transition (/var/runtime/node_modules/dynamodb-doc/node_modules/aws-sdk/lib/request.js:21:12)",
    "AcceptorStateMachine.runTo (/var/runtime/node_modules/dynamodb-doc/node_modules/aws-sdk/lib/state_machine.js:14:12)",
    "/var/runtime/node_modules/dynamodb-doc/node_modules/aws-sdk/lib/state_machine.js:26:10",
    "Request.<anonymous> (/var/runtime/node_modules/dynamodb-doc/node_modules/aws-sdk/lib/request.js:22:9)",
    "Request.<anonymous> (/var/runtime/node_modules/dynamodb-doc/node_modules/aws-sdk/lib/request.js:606:12)",
    "Request.callListeners (/var/runtime/node_modules/dynamodb-doc/node_modules/aws-sdk/lib/sequential_executor.js:104:18)"
    ]
}